The present invention relates to an iterative method of decoding a received signal transmitted in data frames.
The invention relates, in addition, to an iterative method of decoding received signals transmitted in data frames via various channels.
Furthermore, the present invention relates to a device for decoding received signals by means of an iterative method.
The invention also relates to a device for receiving received signals transmitted in data frames with at least one device for decoding the received signals by means of an iterative method.
The invention furthermore relates to a base station of a radio telecommunications system having a multiplicity of mobile-radio units, having at least one further base station, the base stations being in radio contact with the mobile-radio units, and at least one control device for controlling the radio telecommunications system that is in contact with the base stations.
Finally, the present invention relates to a radio telecommunications system having a multiplicity of mobile-radio units, having a plurality of base stations that are in radio contact with the mobile-radio units, and having at least one control device for controlling the radio telecommunications system that is in contact with the base stations.
The invention is based on a priority application EP 01 440 100.4 which is hereby incorporated by reference.
Decoding methods of the type mentioned at the outset are used in the prior art, for example in decoding devices of base stations of radio telecommunications systems. FIG. 2 shows a block diagram of a known so-called turbo decoder, such as is used, for example, for decoding a channel in a node B of UMTS (Universal Mobile Telecommunications System) radio telecommunications systems. More detailed items of information on the known turbo decoder can be found in 3-GPP (3rd Generation Partnership Project) specification 25.212 V.3.4.0, to which reference is expressly made. A received signal applied to the turbo decoder is denoted by the reference symbol 1. The input signal is transmitted in data frames according to a CDMA (code division multiple access) method. It is first fed to a so-called unpuncturing unit 2, which matches the received signal 1 transmitted in the data frames to specified data frames of the decoder. This is done, for example, by adding bits omitted in the frame of the data transmission. The added bits have the amplitude zero. They are treated in the decoding frame as bits having a low reliability and weighted low according to the amplitude. The unpuncturing is part of a so-called rate matching. Generated by the unpuncturing unit 2 are an output signal e and two redundancy signals y1, y2 for the purpose of error correction.
The output signal e and the first redundancy signal y1 are fed to a first so-called constituent decoder 3 having a so-called soft output at the output 4. A soft output means that the entire bandwidth of real numbers can be applied to the output 4 instead of, as in the case of the so-called hard output, where only +1, −1 (physical bits) or 0, 1 (logic bits) may be applied at the output. As a result, errors in the decoding of a transmitted received signal subject to interference are reduced. In the case of the soft output, the bits have various amplitudes corresponding to their reliability and are weighted according to their amplitude.
Used as constituent decoders are, typically, decoders of the MAP (maximum a posteriori) type or, for example of the LogMAP, MaxLogMAP, SOVA or Viterbi types.
The output 4 of the decoder 3 is fed to an interleave unit 5, which rearranges the bits within the data frames so that originally adjacent bits are as far away from one another as possible. In this connection, it is assumed that errors in the transmission of the received signal affect not only individual bits but, as a rule, a multiplicity of consecutive bits. Whereas the decoding of a data frame containing occasionally occurring defective bits is possible in a relatively problem-free manner, decoding of a data frame having a multiplicity of consecutive defective bits is very expensive or even impossible. For this reason, the bits are rearranged by the interleave unit 5 in order to generate a data frame containing defective bits that occur only occasionally and that can readily be decoded despite a transmission error that occurs in an accumulated manner and that affects a multiplicity of consecutive bits.
The output of the interleave unit 5 is fed, together with the second redundancy signal y2, to a second constituent decoder 6. From the point of view of structure, the latter corresponds to the first constituent decoder 3. According to the prior art, the output of the second constituent decoder 6 is regularly fed back eight times via a feedback 7 and a de-interleave unit 8 disposed therein to the input of the first constituent decoder 3. After eight iterations have taken place, the output of the second constituent decoder 6 is emitted via a threshold detector 9 and a further de-interleave unit 10. The output 11 of the further de-interleave unit 10 is the decoded signal (the so-called estimated hard bits).
Turbo decoding involving eight iterations of a received signal that is transmitted via a 384 kbit/s data channel requires a computing capacity of 78×106 instructions/s (78 Mips). At a clock rate of 180 Hz and 60% capacity utilization, a conventional digital signal processor (DSP) of the TigerSHARC type can make available about 108×106 instructions/s (108 Mips). Consequently, according to the prior art, a maximum of 384 kbit/s data channel can be decoded by a DSP.